2. The new, close-up view from space

Volcanoes

Volcanoes on Earth and Venus

Volcanoes, another common aspect of the solar system, are driven by internal heat. For a large rocky planet, internal heat is continuously generated by the slow decay of radioactive material. Satellites can be heated by tidal interaction with their planet. Heat was also provided when the planets and satellites originated, as the result of high-speed collisions between smaller bodies.

Two planets now have a large number of volcanoes - the Earth and Venus. Because of their large size and rocky composition, both planets have internal heat powered by radioactive decay. They have become hot enough inside to melt solid rock into liquid magma that is bottled up within their deep interior. The magma is swollen by heat, becoming lower in density, and rises through the cooler, higher-density material. A volcano is formed and lava flows across the planet's surface. Molten rock trapped beneath a planet's crust is called magma; when molten rock issues from the crater of a volcano or a fissure in the crust it is called lava.

The Hawaiian islands are giant volcanoes, formed when magma moved up from inside the Earth. Mauna Kea and Mauna Loa, on the big island of Hawaii, together form a mountain of lava that is much broader than it is tall; it is more than 200 thousand meters across and rises 9 thousand meters above the ocean floor. Such shield volcanoes have gentle slopes that have been built up from hundreds and even thousands of eruptions and individual flows of highly fluid lava. Mauna Loa is still erupting and growing, with repeated surges of lava that flow down its flanks.

The upwelling of pent-up heat and magma also forms rift valleys on Earth, with steep sides, sunken floors, and copious outpouring of lava. An example is the Great Rift Valley in Africa, a long forking gash that crosses 4.5 million meters of the continent. It extends from Mozambique in the south to Ethiopia in the north, branching out through the Red Sea in one direction and diverging through the Gulf of Aden in another.

Tens of thousands of shield volcanoes have been identified on the face of Venus, by their round shapes and gentle slopes. They range in size from major, Hawaii-sized edifices that are hundreds of thousands of meters across to more numerous, smaller domes that pop up everywhere on the surface. These shield volcanoes have been built up from runny lava that spreads out over great distances with the ease of spilt olive oil.

A smaller number of volcanic flows on Venus appear to be built from lava that is as stiff and thick as batter. In places, the sluggish lava has oozed onto the hot, flat surface of Venus, forming volcanic domes as round and flat as pancakes. Each one has a dark feature almost precisely at the center, suggesting a vent from which the pasty lava flowed, like pancake batter on a hot griddle.

Volcanoes on Mars

The large volcanoes on Mars have the gentle slopes and round shapes of shield volcanoes on Earth, but the volcanoes on Mars stand higher. A striking example is Olympus Mons - Mons is a Latin term for ""mountain"". Olympus Mons is very much larger than any volcano on Earth, and has been active over a long period of time. Rising to a height of at least 23 thousand meters above the surrounding plains, it has a diameter of about 600 thousand meters at its base, giving it a volume as much as one hundred times that of any large terrestrial shield.

Another type of large volcanic structure on Mars is the tholus, which is similar to the shield type of volcano but with somewhat steeper slopes, perhaps due to eruptions of more viscous lava or to a lower eruption rate. The term tholus designates ""a small domical mountain or hill"". An example is Ceraunius Tholus, with an estimated age of about 2.4 billion years.

A third kind of Martian volcano is the patera, with an exceptionally low summit and complex caldera. The name patera is Latin for ""shallow dish or saucer"". They sometimes exhibit the worn-down appearance of old age. Images of the Martian surface suggest that volcanic activity has persisted from the planet's youth into relatively recent times. Like the Moon and Mercury, the red planet bears the scars of a steady rain of meteorites. Relative ages of volcanoes and lava flows can be determined from the density of impact craters on them. The paucity of impact craters near the top of Olympus Mons suggests that the latest lava flows are relatively young, whereas the higher crater density on the volcano's flanks and outer edges indicate a ripe old age for the edifice itself.

Close-up images of other volcanic landforms, taken with cameras on Mars Global Surveyor in 1999-2000, indicate a distinct lack of craters and a fresh, young surface. The impact crater densities on some lava flows in the Martian plains, within the Elysium Planitia region, are up to a thousand times less than those on the lunar maria. The low crater density indicates that volcanism is a continuing process in the recent geologic history of Mars, within the past 100 million years and probably even the last 10 million years. The presence of young lava flows and volcanoes implies that Mars may still be volcanically active today.

Active volcanoes on Jupiter's satellite Io

There is one place in the solar system that is now more volcanically active than any other place; it is Jupiter's satellite Io. It is the hottest satellite in the solar system, so hot that you can see it melting before your eyes. Io is now spewing out 100 times more lava than all the volcanoes on the Earth. This is a totally unexpected discovery, made by the inquisitive camera eyes of Voyager 1 in 1979. Volcanoes are literally turning the satellite inside out, so parts of Io's surface are younger than your backyard.

Galileo returned for a close-up view of Io's volcanoes in 1999-2000, providing a better understanding of the sizzling world. Instruments on Galileo measured the temperatures of the volcanoes, showing that the lava is at 1700 to 2000 degrees kelvin, up to twice the temperature of volcanoes on Earth. The high-temperature eruptions emit gaseous sulfur and sulfur dioxide; the bright surface flows are attributed to sulfur and the white surface deposits to sulfur dioxide. The very high temperatures apparently rule out liquid sulfur as a dominant volcanic fluid, and they have certainly driven off any water that might have been on Io.

Volcanism on Neptune's satellite Triton

Neptune's largest satellite, Triton, is the coldest moon ever recorded, with a temperature of just 38 degrees kelvin, approaching absolute zero where all motion stops. It is so cold because Neptune is so far away from the Sun, therefore receiving little sunlight, and also because Triton reflects more of the incident sunlight than most satellites - only Enceladus and Europa are comparable. Yet, the frozen moon is a dynamic, alive world, set in motion and molded by volcanic eruptions.

Numerous dark plumes and streaks, found in the midst of the bright southern cap of Triton, suggest a different kind of volcanic activity, propelled by relatively recent eruptions of nitrogen gas. Nitrogen boils at very low temperatures, at just 77 degrees kelvin on Earth, and when it boils it expands, producing enormous pressures that can shoot gas and other material high into Triton's thin nitrogen atmosphere. Thus, geyser-like eruptions may have lofted the dark material outward from beneath the surface. The prevailing winds would then carry it across the satellite, depositing it on the ice as dark streaks.